This invention relates to a method and assembly for minimizing spurious interference effects and minimizing the recording of secondary gratings in the exposure of holograms to produce hologram optical elements. The assembly comprises a glass base plate, on which is supported a photosensitive holographic exposure material. The holographic material can be a gelatin based material, such as dichromated gelatin, silver halide emulsion, or diazo gelatin. On top of that is a glass cover plate. Both of the glass plates are anti-reflection coated. Sandwiched between the gelatin holographic exposure material and the cover plate is a solid, transparent and removable film which provides a rigid structure.
In the preparation of holographically exposed optical elements, it is desired to eliminate secondary gratings which are caused by reflections from adjacent optical surfaces. The secondary gratings cause ghost images or undesirable spectral flare in the exposed optical element and thus result in degradation of its performance. The responsible reflections can be minimized by putting the photosensitive gelatin holographic exposure material between anti-reflection coated glass. However, the glass pieces must be rigidly held in place during exposure. In the past, spurious interference patterns were eliminated by using very thick glass plates, when the exposure geometry would permit. A publication defining this system is found at T. A. Shankoff and R. K. Curran, "Efficient, High Resolution, Phase Diffraction Gratings", Applied Physics Letters 13, 239-241 (1968).
An index matching liquid is necessary between the upper glass plate and the gelatin exposure material, and unfortunately this system presents severe difficulties and limitations. For example, if the plates are stood up on edge, the index matching liquid has a tendency to run out and leave an air gap between the plates. Another difficulty appears in the dimensional instability of the two glass plates. If the cover plate moves in relation to the base plate on which the gelatin exposure material is mounted, the phase of the light passing through to the exposure material is changed. A phase change of 180.degree. (or a plate movement of about 1/2 of a light wave length) can completely destroy the quality of the exposed hologram. Movement of the plates is very hard to prevent since the plates are usually held in place by two opposite forces: gravity and the surface tension of the index matching liquid between the two plates. Systems in which the plates are held together by a mechanical structure are subject to mechanical creep and instability.
Thus there is need for a method and assembly for holographic exposure so that holographic optical elements can be produced with minimized spurious gratings and with minimized exposure problems.